# Longest Palindromic Substring in Python

Suppose we have a string S. We have to find the longest palindromic substring in S. We are assuming that the length of the string S is 1000. So if the string is “BABAC”, then the longest palindromic substring is “BAB”.

To solve this, we will follow these steps

• Define one square matrix of order same as the length of string, and fill it with False
• Set the major diagonal elements as true, so DP[i, i] = True for all i from 0 to order – 1
• start := 0
• for l in range 2 to length of S + 1
• for i in range 0 to length of S – l + 1
• end := i + l
• if l = 2, then
• if S[i] = S[end - 1], then
• DP[i, end - 1] = True, max_len := l, and start := i
• otherwise
• if S[i] = S[end - 1] and DP[i + 1, end - 2], then
• DP[i, end - 1] = True, max_len := l, and start := i
• return a substring of from index start to start + max_len

## Example(Python)

Let us see the following implementation to get better understanding −

Live Demo

class Solution(object):
def longestPalindrome(self, s):
dp = [[False for i in range(len(s))] for i in range(len(s))]
for i in range(len(s)):
dp[i][i] = True
max_length = 1
start = 0
for l in range(2,len(s)+1):
for i in range(len(s)-l+1):
end = i+l
if l==2:
if s[i] == s[end-1]:
dp[i][end-1]=True
max_length = l
start = i
else:
if s[i] == s[end-1] and dp[i+1][end-2]:
dp[i][end-1]=True
max_length = l
start = i
return s[start:start+max_length]
ob1 = Solution()
print(ob1.longestPalindrome("ABBABBC"))

## Input

"ABBABBC"

## Output

"BBABB"